1. Field of the Invention
The present invention relates to a system for measuring sectional shape of a specimen surface. More specifically, the invention relates to a method of measuring sectional shape maintaining high precision in the measurement of a sectional shape of a specimen using a scanning electron microscope (hereinafter referred to as "SEM") without being affected by differences in the material of specimen, by an edge effect or by a shadow formed on an uneven surface of the specimen, and further relates to a system therefor.
2. Description of the Prior Art:
It has recently been attempted extensively to reconstruct a surface shape based upon the brightness information of an image. One of the examples using SEM is found in the Journal of Electron Microscopy, "Measurement of Surface Topography Using SEM with Two Secondary Electron Detectors", Vol. 34, No. 4, 1985, pp. 328-337.
According to this system, two detectors are attached to the SEM, and the gradient on one line is detected, i.e., a differentiated value is found in a direction which couples the detectors, and is integrated from the end to find the sectional shape on the line, by utilizing the relationship that the difference between square powers of the two signals of the detectors varies nearly in proportion to the gradient component in the direction which couples the detectors of the surface elements.
If signals of the right side and left side detectors are denoted by I.sub.R and I.sub.L, signals of the right and left detectors of a flat part used as normalization values are denoted by I.sub.Rn and I.sub.Ln, and a constant for correction of nearly 1 is denoted by k, then the gradient A of the surface element is given by ##EQU1##
Here, the direction coupling the detectors is represented by an X-axis, the direction that meets at right angles therewith on the sample surface is represented by a Y-axis, and the direction of height is represented by a Z-axis. If a gradient at a point X=j is A.sub.j when a line Y=Y.sub.O is being scanned, the height Z.sub.j of X=j is given by the following equation. ##EQU2##
Between the SEM and a general optical system, the direction of an electron gun can be corresponded to the viewing direction or the direction of an imaging system, and the direction of the detectors can be corresponded to the direction of a light source. That is, the path consisting of electron gun.fwdarw.specimen.fwdarw.detector in the SEM can be replaced by imaging system.rarw.specimen.rarw.light source being corresponded to the direction in which light travels that is just opposite to the direction in which the electrons travel. Therefore, when the image is picked up using the light sources in a direction in which they are opposed to each other, the same processing as the above processing can be effected relying upon two brightnesses at the same point.
In the above-mentioned conventional art, however, consideration had not been given to the case where different materials exist on the specimen or peculiar effects are exhibited depending upon the shape, or in regard to the change of the values I.sub.R and I.sub.L due to the formation of shadow caused by an uneven surface of the specimen.
According to the above-mentioned conventional art, a shadow forms on the specimen surface when different materials exist on the specimen, when there exists a peculiar effect in the brightness at a characteristic point such as an edge, or when there exists great unevenness on the specimen; i.e., the right and left detectors produce different signals making it difficult to obtain a correct gradient component. As a result, the sectional shape is obtained incorrectly.